The present invention relates to a piezoelectric transformer device, e.g., a Rosen type piezoelectric transformer device having a multilayer structure.
Recently, liquid crystal display panels have been widely used as display units in notebook personal computers which are easy to carry. Such a liquid crystal display panel incorporates a cold cathode tube serving as a backlight for backlighting the panel. In order to light this cold cathode tube, a high voltage of about 1 kV or more is required. To maintain emission of light, a voltage of about several hundred volts must be applied.
In such notebook personal computers and similar products, owing to their characteristics, the demand for compact, small-power-consumption boosting modules for backlights is high. In order to meet this demand, a so-called Rosen type piezoelectric transformer device having a single-plate structure is used. According to a power supply module using a piezoelectric transformer device having a single-plate structure, reductions in size and weight of the module can be attained as compared with a winding transformer having a similar output capacity.
However, in a piezoelectric transformer device having a single-plate structure having, for example, a length of about 30 mm, a width of about 3 mm, and a thickness of about 2 mm, the boosting ratio (the ratio of the output voltage to the input voltage) is about 4 to 6 at a heavy load, e.g., a 2-W output. That is, a desired boosting ratio cannot be obtained. For this reason, a compact winding transformer must be connected to the input stage of the piezoelectric transformer device.
In recent years, piezoelectric transformer devices having multilayer structures which attain large boosting ratios have been proposed in, for example, Japanese Patent Laid-Open Nos. 4-338685, 7-79027, and 7-131088. Such a piezoelectric transformer device having a multilayer structure is equivalent to a plurality of piezoelectric transformer devices having single-plate structures which are connected in parallel with each other, and is designed to obtain a high boosting ratio. For example, this device has an input region in which ceramic piezoelectric members and inner electrodes, each having a thickness of about 50 to 300 .mu.m, are alternately stacked on each other. These stacked inner electrodes are alternately and electrically connected to each other to constitute two inner electrode groups electrically insulated from each other. These two inner electrode groups are electrically connected to two outer electrodes formed on the upper and lower surfaces of the input region (Japanese Patent Laid-Open No. 7-302938).
FIGS. 1 to 5 show an example of the structure of a piezoelectric transformer device having a multilayer structure.
FIG. 1 is a perspective view showing a piezoelectric transformer device having a general multilayer structure. FIG. 2 is a front view of the piezoelectric transformer device in FIG. 1. FIG. 4 is a plan view of the piezoelectric transformer device in FIG. 1.
This piezoelectric transformer device has an outer shape like the one shown in FIGS. 1, 2, and 4. The right and left half regions of the device will be respectively referred to as the second and first regions hereinafter.
Outer electrodes 101 and 102 as primary (input) electrodes are formed on the upper and lower surfaces of the first region. A plurality of inner electrodes 101a and 102a are alternately stacked between the outer electrodes 101 and 102, and the spaces between these inner electrodes are filled with a piezoelectric element 106. An outer electrode 103 as a secondary (output) electrode is formed on the right end portion of the second region. Lead wires 105 are connected to the outer electrodes 101, 102, and 103 with solder portions 104.
The internal structure of the first region will be described next. FIG. 3 is a sectional view taken along a line A-A' of the piezoelectric transformer device in FIG. 2. FIG. 5 is a sectional view taken along a line B-B' of the piezoelectric transformer device in FIG. 1.
As shown in FIGS. 3 and 5, in the first region, the plurality of inner electrodes 101a are connected to each other through a columnar conductor (to be referred to as an interlevel connection conductor hereinafter) 107a, and the plurality of inner electrodes 102a are connected to each other through an interlevel connection conductor 107b. Circular holes (to be referred to as holes hereinafter) are formed in the inner electrodes 101a and 102a so that the inner electrodes 101a and 102a are not connected to each other through the interlevel connection conductors 107aand 107b.
The piezoelectric transformer device having the above multilayer structure is driven as follows. First of all, a high voltage is applied between the outer electrodes 101 and 102 to polarize the first region in the vertical direction (thickness direction). A predetermined voltage is then applied between the outer electrode 101 or 102 and the outer electrode 103 to polarize the second region in the longitudinal direction. When an AC voltage is applied between the outer electrodes 101 and 102 of the device in the polarized state, the piezoelectric element 106 in the first region mechanically vibrates in accordance with the piezoelectric material constant unique to the piezoelectric element, the resonance characteristics, and the dimensions of the overall device. The vibrations are converted into a voltage by the piezoelectric element 106 in the second region. As a result, a boosted high AC voltage can be extracted from the outer electrode 103.
When the present inventors manufactured a multilayer type piezoelectric transformer device having a length of 30 mm, a width of 3 mm, and a thickness of 2 mm by stacking 25 piezoelectric members each having a thickness of about 80 .mu.m, and conducted a test on the device, a very high boosting ratio, about 80, was obtained at 2-W output.
In the piezoelectric transformer device having the above structure, however, since a plurality of inner electrodes are exposed on the side surfaces of the piezoelectric transformer device, electric discharges may occur between the inner electrodes exposed on the side surfaces to cause breakdown, resulting in imperfect polarization, especially when the piezoelectric transformer device is polarized in the air. For this reason, the present inventors has proposed a piezoelectric transformer device having the following multilayer structure in Japanese Patent Application No. 8-52553. In this structure, the inner electrodes are not exposed on the side surfaces of the device, and holes formed in advance in the piezoelectric members at the positions where the inner electrodes are to be stacked are filled with conductors so as to electrically connect the inner electrodes to each other.
In the conventional piezoelectric transformer device shown in FIG. 1, however, the outer electrodes to which the lead wires 105 are to be soldered are formed on the three surfaces, i.e., the upper surface, lower surface, and longitudinal end face of the device. That is, the outer electrodes are not in the same plane. For this reason, in an automatic assembly process using industrial robots, the soldering step and the like are complicated.
In addition, the above conventional piezoelectric transformer device has the structure in which the inner electrodes an the outer electrodes are exposed on the surfaces of the device. For this reason, the piezoelectric transformer device must be housed in a case (not shown) consisting of, e.g., an insulating material, similar to a piezoelectric transformer device having a single-plate structure, in consideration of safety. A case much larger than the piezoelectric transformer device having such a structure is required to house the device in consideration of the volume of the connecting portion of the device. The manufacturing step of housing the device in the case itself complicates the automatic assembly process.
Further, in order to extract a boosted high AC voltage from the piezoelectric element 106 having the multilayer structure, the outer electrode 101 or 102 and the outer electrode 103 are used. Unlike a general winding transformer, therefore, the outer electrode 101 or 102 is used as a common electrode shared by the primary (input) and secondary (output) sides. For this reason, the piezoelectric transformer device having the multilayer structure described above cannot be use for a circuit that demands electric insulation between the primary and secondary sides.
Furthermore, the conversion efficiency of the piezoelectric transformer device having the multilayer structure, i.e., the value obtained by dividing the output power by the input power, is 70 to 80%, which is lower than the conversion efficiency (90% or more) of the piezoelectric transformer device having the single-plate structure.
In a FERROELECTRICS, 1990, UK, vol.101, ISSN0015-0193, pages 193-200, a monolithic piezoelectric transformer device using PZT-G ceramics is disclosed. In the document, the transformer device comprises a primary side (driving section) in which ceramic piezoelectric sheets and internal electrodes for poling and operating are alternately stacked on each other and sintered at about 960.degree. C. In the primary side of the transformerdevice, alloy of Ag/Pd=80/20 is used as internal electrodes.
In a PATENT ABSTRACTS OF JAPAN, vol. 015, no. 200 (E-1070), May 22, 1991 & JP-A-03 054878, a piezoelectric transformer device having external electrodes with pairs of high and low impedance parts is disclosed. In the transformer device, the external electrodes are connected to every two electrode layers which are laminated alternately, with piezoelectric layers. Alloy of Ag--Pg is used in internal electrodes of the transformer device.
U.S. Pat. No. 5,278,471 discloses a piezoelectric ceramic transformer including a high impedance portion which has internal electrode layers which are opposite each other in a thickness direction in a radial central portion of a piezoelectric ceramic disk. In the ceramic transformer, alloy of Ag--Pg is also used in the internal electrode layers of the transformer.
In a PATENT ABSTRACTS OF JAPAN, vol. 096, no. 007, Jul. 31, 1996 & JP-A-08 069890, Mar. 12, 1996, a multilayer piezoelectric transformer comprising alternately layered piezoelectric bodies is disclosed. In the transformer, input external electrodes are respectively connected to layered input internal electrodes.